1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display storage device and a method of fabricating the same.
2. Background of the Related Art
In general, a liquid crystal display (LCD) device displays an image corresponding to data signals that are individually applied to LCD cells arranged in a matrix form. Thus, the LCD cells adjust light transmissivity of each of the LCD cells.
An LCD device includes a liquid crystal panel upon which a plurality of LCD cells constituting pixel units are arranged, thereby forming an active matrix portion and a driver integrated circuit (IC) portion for driving the LCD cells. The LCD device includes a common electrode formed on a first one of opposing inner faces of upper and lower substrates in the liquid crystal panel, pixel electrodes formed on a second one of opposing inner face of the upper and lower substrates so as to confront the common electrode, and a liquid crystal material layer formed between the upper and lower substrates. An electric field is applied to the liquid crystal material layer by providing a potential to the common electrode and the pixel electrodes. Each of the pixel electrodes are disposed at each of the LCD cells formed on the lower substrate, and the common electrode is disposed upon an entire surface of the upper substrate.
A plurality of data lines and a plurality of gate lines are perpendicularly formed on the lower substrate. The plurality of data lines transfer data signals from a data driver IC to corresponding ones of the LCD cells, and the plurality of gate lines transfer scan signals from a gate driver IC to corresponding ones of the LCD cells. Accordingly, each of the LCD cells are defined by an intersection of one of the data lines and one of the gate lines. The gate driver IC sequentially applies the scan signals to the gate lines to sequentially select the gate lines of the LCD cells, and the data driver IC supplies the LCD cells of the selected gate line with one of the corresponding data signals.
A thin film transistor is formed within each of the LCD cells, and functions as a switching device. A conductive channel is formed between source/drain electrodes of the thin film transistor when a gate electrode of the thin film transistor receives the scan signal through a corresponding one of the gate lines.
FIG. 1 is a plan view of an LCD cell of a liquid crystal display according to the conventional art. In FIG. 1, an LCD cell is formed at an intersection between a data line 2 and a gate line 4. The LCD cell includes a thin film transistor TFT and a pixel electrode 14 that is connected to a drain electrode 12 of the thin film transistor TFT. A source electrode 8 of the thin film transistor TFT is connected to the data line 8, a gate electrode 10 of the thin film transistor TFT is connected to the gate line 4, and the drain electrode 12 of the thin film transistor TFT is connected to the pixel electrode 14 through a drain contact hole 16.
The thin film transistor TFT includes an active layer (not shown) for forming a conductive channel between the source electrode 8 and the drain electrode 12 by application of a scan signal to the gate electrode 10 via the gate line 4. As the conductive channel is formed, a data signal transmitted on the data line 2 is supplied to the drain electrode 12 via the source electrode 8. Accordingly, the data signal is transmitted to the pixel electrode 14, and together with a potential applied to the common electrode (not shown), generate an electric field to the liquid crystal material layer (not shown). Once the electric field is applied to the liquid crystal material layer, the liquid crystal molecules rotate by dielectric anisotropy to transmit light emitted from a backlight device toward the upper substrate through the pixel electrode 14. Thus, an amount of the transmitted light is controlled by a voltage value of the data signal.
A storage capacitor 18 includes a storage electrode 20 formed on the gate line 4 to connect to the pixel electrode 14 through a storage contact hole 22. A gate insulating layer (not shown) is formed between the storage electrode 20 and gate line 4 to electrically isolate them from each other. Accordingly, the gate insulating layer is formed during the fabrication process of forming the thin film transistor TFT.
The storage capacitor 18 is charged with a voltage value of the scan signal for an amount of time taken to apply the scan signal to the gate line 4 of a previous LCD cell. Then, the storage capacitor 18 discharges the charged voltage while the voltage value of the data signal is applied to the pixel electrode 14 as the scan signal is applied to the gate line 4 of a next LCD cell. Thus, a voltage variation of the pixel electrode 14 is minimized.
FIGS. 2A to 2G are cross sectional views of a fabrication process of a liquid crystal display along I-I in FIG. 1 according to the conventional art. In FIG. 2A, a metal material, such as Mo, Al, or Cr, is deposited upon a lower substrate 1 by a sputtering process. Then, the metal material is patterned by a photolithographic process to form a gate electrode 10.
In FIG. 2B, an insulating material, such as SiNx, is deposited on an entire surface of the lower substrate 1 including the gate electrode 10, thereby forming a gate insulating layer 30.
In FIG. 2C, an amorphous silicon semiconductor layer 34 is formed on the gate insulating layer 30, and an ohmic contact layer 32 is formed on the semiconductor layer 34. Then, an active layer 36 of a thin film transistor (TFT) is formed by patterning the ohmic contact layer 32 and the semiconductor layer 34.
In FIG. 2D, a metal material is deposited on the gate insulating layer 30 and the ohmic contact layer 32. Then, the metal material is patterned to form a source electrode 8 and a drain electrode 12 of the TFT. In addition, a portion of the semiconductor layer 34 is exposed between the source and drain electrodes 8 and 12.
In FIG. 2E, a passivation layer 38, such as SiNx, is deposited on an entire surface of the gate insulating layer 30, the source and drain electrodes 8 and 12, and the exposed portion of the semiconductor layer 34 by a chemical vapor deposition (CVD) process. In order to improve an aperture ratio of the LCD cell, low dielectric constant organic materials, such as benzocyclobutene (BCB), sin on glass (SOG), and acryl, are commonly used as the passivation layer 38.
In FIG. 2F, a drain contact hole 16 is formed by etching a portion of the passivation layer 38 above the drain electrode 12, thereby exposing a portion of the drain electrode 12.
In FIG. 2G, transparent electrode material is deposited on the passivation layer 38 by a sputtering process, and patterned to form a pixel electrode 14. The pixel electrode 14 is connected to the drain electrode 12 through the drain contact hole 16.
FIGS. 3A to 3D are cross sectional views of the fabrication process of a liquid crystal display along II-II in FIG. 1 according to the conventional art. In FIG. 3A, a gate line 4 is patterned simultaneously with patterning of the gate electrode 10 (in FIG. 2A) upon a lower substrate 1, and a gate insulating layer 30 is formed on the lower substrate 1 including the gate line 4.
In FIG. 3B, a storage electrode 20 is patterned on the gate insulating layer 30 to form an upper electrode of the storage capacitor 18. The patterning of the storage electrode 20 is simultaneously performed with formation of the source and drain electrodes 8 and 12 (in FIG. 2D). A portion of the storage electrode 20 overlaps a portion of the gate line 4 with the gate insulating layer 30 therebetween.
In FIG. 3C, a passivation layer 38 is formed upon the gate insulating layer 30 and the storage electrode 20. Then, a storage contact hole 22 is formed by etching a portion of the passivation layer 38 overlying the storage electrode 20, thereby exposing a portion of the storage electrode 20 through the storage contact hole 22. The passivation layer 38 overlying the storage electrode 20 is formed simultaneously with formation of the passivation layer 38 (in FIG. 2E), and the storage contact hole 22 is formed simultaneously with formation of the drain contact hole 16 (in FIG. 2F).
In FIG. 3D, a pixel electrode 14 is patterned upon the passivation layer 38 to electrically connect to the storage electrode 20 through the storage contact hole 22. The pixel electrode 14 contacting the storage electrode 20 is formed simultaneously with formation of the pixel electrode 14 contacting the drain electrode 12 (in FIG. 2G).
FIG. 4 is a diagram showing an effect of a liquid crystal material layer by application of a DC electric field to a gate line. In general, an AC voltage is applied to the data line 2 and the pixel electrode 14, and a low level DC voltage is uniformly applied to the gate line 4 to drive a unit LCD cell. The low level DC voltage applied continuously to the gate line 4 degrades the liquid crystal characteristics within the unit LCD cell area formed over the gate line 4. In addition, the low level DC voltage adversely influences the driving of the LCD cell, thereby generating unwanted afterimage.
In FIG. 4, a liquid crystal material layer 53 is formed between a lower plate 51 and an upper plate 52 of a liquid crystal display device. Once a DC voltage is applied to a gate line 54 patterned upon the lower plate 51, the liquid crystal material layer 53 disposed over the gate line 54 is adversely influenced by a DC electric field. Accordingly, characteristics of liquid crystal molecules in the liquid crystal material layer 53 are degraded, and driving of a corresponding LCD cell is adversely affected, thereby generating the unwanted afterimage.
FIG. 5 is another diagram showing of an effect of a liquid crystal material layer by application of a DC electric field to another gate line. In FIG. 5, a storage electrode 55 is formed as an upper electrode of a storage capacitor on a gate line 54 patterned on a lower plate 51. Accordingly, most of the DC electric field is concentrated between the gate line 54 and storage electrode 55. Thus, a strength of the DC voltage applied to a liquid crystal material layer 53 is attenuated, thereby preventing degradation of the liquid crystal molecules of the liquid crystal material layer 53.
However, to ensure overlap of the storage electrode 55 with the gate line 54, an area of the storage electrode 55 should be increased, thereby increasing electrical charge capacity and stabilizing a display image. Disadvantageously, the scan signal applied to the gate line 54 becomes delayed.